In general, output signals of a loudspeaker may have a nonlinear relationship with input signals to the loudspeaker. In other words, there is nonlinear distortion in the output of the loudspeaker. Various methods have been proposed to estimate the nonlinear distortion in the output of the loudspeaker. According to one kind of the methods, a multi-tone test signal is used to estimate the nonlinear distortion. The multi-tone test signal is the sum of several sinusoidal waves whose frequencies are typically distributed logarithmically across the audio frequency range, which is considered to be similar to the spectrum of musical signals. An example of such methods can be found in Richard C. Cabot et al., “METHOD AND APPARATUS FOR FAST RESPONSE AND DISTORTION MEASUREMENT,” U.S. Pat. No. 5,748,001.